This invention relates to an improved process for producing lithium soap greases containing a borate salt and having a high dropping point.
Lithium greases have been known and widely used for many years. The lithium soaps that are used as thickening agents for these greases are ordinarily prepared by the reaction of lithium hydroxide or other suitable lithium base with a conventional high molecular weight acid or acids. The principal advantages of lithium greases have been high water resistance and ease of dispersion of the soaps in all types of lubricating oil base stocks. Particularly useful have been greases prepared from lithium hydroxystearate, since the soaps of the hydroxystearic acids and related hydroxy fatty acids have been found to be more mechanically stable than the corresponding soaps of the conventional fatty acids.
There are many fields of application for grease compositions where a high dropping point is required, as for example, in the lubrication of traction motor bearings. The bearings of these locomotives may be required to operate for periods of as much as three years without any maintenance, and temperatures as high as 250.degree. F. (121.degree. C.) can be reached in such bearings. Other applications require even higher dropping points in the grease composition.
Several teachings exist for preparing high dropping point greases e.g. U.S. Pat. No. 2,940,930 where mixtures of monocarboxylic and dicarboxylic acids are used. U.S. Pat. Nos. 3,223,633 and 3,223,624 teach the preparation of high dropping point greases from a three component mixture of acids. U.S. Pat. No. 3,681,242 discloses a particular method of preparing lithium soap greases containing a dilithium soap of a dicarboxylic and wherein a two stage heating operation is used. U.S. Pat. No. 3,985,662 discloses a high dropping point grease derived from a lithium soap of a fatty acid which contains an epoxy group and/or ethylenic unsaturation.
U.S. Pat. No. 3,758,407 discloses a high dropping point lithium soap grease which comprises a lithium soap of C.sub.12 to C.sub.24 hydroxy fatty acid and a monolithium salt of boric acid.
Despite all the noted disclosures of high dropping point greases there still is the need for improved compositions and/or processes for developing greases with increased high dropping points.